The lamellae are the most important contributors to the respiratory surfaces of fish (Evans et al., 2005; Nilsson, 2007). T. lalius had morphological variations in the filaments and lamellae during acidic treatment. No visible lamellae can be found in approximately 30% of our samples from the 4-day acidic group. A similar phenomenon was observed in the South American obligate air-breathing fish Arapaima gigas, which has no visible lamellae in the adult stage (Brauner et al., 2004). Gill variations in Arapaima gigas can be seen, however, during its development stages rather than as a response to the environment. In the present study, the morphological modification in T. lalius is a result of acidic acclimation. We can therefore conclude that the gill structures of aquatic air-breathing fish